From its first gleaming razor to its last slashed throat, Cygnet Theatre’s “Sweeney Todd” may not be pretty, but it is pretty great. Bloody great, actually.

Cygnet shines a beam of bleak inspiration into every dark corner of the Stephen Sondheim musical about a tormented barber with mayhem on the brain. With a top-flight cast, excellent live musicians and a staging that’s steeped in the work’s monumental creepiness, this production thrills, and (it has to be said) kills.

“Sweeney” happens to be the biggest show in Cygnet’s seven-year history; it also should rank as the company’s most complete success to date (no small feat), and a production that likely will be talked about for a good while to come.

There’s no telling why Sondheim thought a musical about a Victorian-era barber with anger-management issues and a taste for queasy cuisine (foodies might die for those mystery-meat pies) would make sense as a musical.

But the composer-lyricist, with the show’s writer, Hugh Wheeler, cooked the operatic slasher piece into a hit when “Sweeney Todd” arrived on Broadway in 1979. The story, adapted from Christopher Bond’s 1973 play, draws on a long line of pulp revenge tales, but it’s Sondheim’s haunting and complex score that really stirs the senses.

Cygnet has cast some of the best voices in town to match those 30-odd musical numbers — and yet, in the lead role of Sweeney, an actor not necessarily known as a vocalist. That would be Sean Murray, who is also the show’s co-director (with James Vasquez), as well as the artistic chief of Cygnet.

Murray brings layers of desperation and festering rage to his portrayal of the Londoner who has spent 15 years in an Australian lockup, while the corrupt and lecherous judge who sent him there violated his wife and stole his young daughter.

It’s not the first vocal role for Murray; two years ago, he also sang in Cygnet’s previous Sondheim show, “A Little Night Music.” His voice was competent enough in that piece; here, his pleasing tenor finds fresh depths, and lends an extra tinge of sympathy to the wronged and wrathful Todd.

It’s all Murray can do, though, to match Deborah Gilmour Smyth, who has what just could be a career turn (again, no small feat) as the jolly pie-maker and Todd’s partner in crime, Mrs. Lovett.

From the character’s first signature number, “The Worst Pies in London,” Smyth, a longtime light at Lamb’s Players Theatre, owns most of the scenes she’s in. Her portrayal, in a role that Angela Lansbury originated on Broadway, is like a macabre carnival of quirks and tics and loopy sweetness. She sings with Cockney-laced brio, and keeps a mean beat with a rolling pin besides.

After a wow of a prologue that sets the show’s harrowing (and wickedly humorous) tone — and has a blood-spattered Todd bursting with a spooky flourish from a trap door in the floor — it’s Mrs. Lovett who first recognizes the man as her long-ago neighbor, the former Benjamin Barker.

She tells him of his family’s fate, though leaving out a crucial detail here and there. If there’s one quibble with Murray’s performance, it’s that his Todd seems less shattered here than he might be for a man whose dream of seeing his kin again is all that’s kept him alive.

Todd vows revenge on the venal Judge Turpin (Steve Gunderson) and his dastardly dandy of a henchman, The Beadle (an icy Geno Carr).

“You’ll soon drip precious rubies,” Todd sings ominously to his silver razors in “My Friends.” But when the plan falters, he becomes an equal-opportunity ghoul, giving fatally close shaves to random clients and dispatching them through the trap door (an effect that earned its own applause on opening night).

For good measure, he and Lovett turn brutality into business, turning out meat pies that bring new meaning to serving one’s customers.

Murray and Vasquez (a veteran director and choreographer) preside over a staging that feels beautifully in tune with Sondheim’s odd mix of the ghastly and the fascinating. Elegiac as it may be, this “Sweeney” bursts with theatrical life.